Bipolar transistor with raised extrinsic self-aligned base using selective epitaxial growth for bicmos integration

ABSTRACT

High performance bipolar transistors with raised extrinsic self-aligned base are integrated into a BiCMOS structure containing CMOS devices. By forming pad layers and raising the height of an intrinsic base layer relative to the source and drain of preexisting CMOS devices and by forming an extrinsic base through selective epitaxy, the effect of topographical variations is minimized during a lithographic patterning of the extrinsic base. Also, by not employing any chemical mechanical planarization process during the fabrication of the bipolar structures, complexity of process integration is reduced. Internal spacers or external spacers may be formed to isolate the base from the emitter. The pad layers, the intrinsic base layer, and the extrinsic base layer form a mesa structure with coincident outer sidewall surfaces.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.11/680,163, filed Feb. 28, 2007 the entire content and disclosure ofwhich is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to semiconductor devices, and moreparticularly to BiCMOS integrated circuits having a raised extrinsicbase formed by selective epitaxy and a mesa structure that includes thebase.

BACKGROUND OF THE INVENTION

A key challenge in integrating high performance bipolar transistors intoa BiCMOS circuit, as is utilized in high performance mixed signalapplications, is to form high performance bipolar transistors withoutadversely affecting the performance of CMOS transistors and withoutintroducing excessive process complexity during the manufacturing. Whilevarious methods of manufacturing bipolar transistors have been known inthe art, not all of them can be employed in BiCMOS circuits since manyof them are incompatible or substantially affect the performance of CMOSdevices adversely. Only integration schemes that fully protect theperformance CMOS devices can successfully integrate high performancebipolar transistors with CMOS devices without degradation of CMOScircuit performance.

To achieve high performance in a bipolar transistor, factors affectingthe critical performance parameters of the bipolar transistor need to beconsidered such as the unit current gain frequency (f_(T)), which is thefrequency at which the current gain becomes 1, and the maximumoscillation frequency (f_(MAX)), which is the maximum frequency at whichthere is still power gain. The two performance parameters, f_(T) andf_(MAX,) critically depend on parasitic parameters of the bipolartransistor structure. The unit current gain frequency is inverselyproportional to the product of base transit time (t_(b)) andcollector-base capacitance (C_(cb)), that is, f_(T) ∝ 1/(t_(b)×C_(cb)).Since the base transit time increases with the thickness of theintrinsic base, high temperature must be avoided to minimize the thermalbroadening of the intrinsic base. The maximum oscillation frequency isproportional to the square root of the unit current gain frequency andis inversely proportional to the produce of base resistance (R_(b)),which is the sum of both intrinsic and extrinsic resistance, andcollector-base capacitance (C_(cb)), i.e., f_(MAX) ∝ (f_(T)/(R_(b)×C_(cb)))^(0.5). To increase f_(MAX), f_(T) needs to be increasedand both R_(b) and C_(cb) need to be decreased. Self-alignment of anextrinsic base to an emitter is thus preferred to reduce the baseresistance, R_(b), and consequently, to increase f_(MAX).

Formation of raised extrinsic base in prior art bipolar transistorstypically employs a chemical mechanical planarization (CMP) process.However, integration of raised extrinsic base bipolar transistors withCMOS devices in a BiCMOS circuit faces challenges since the patternedgate electrodes of CMOS devices introduces topographical variations,that is, differences between the height of the bipolar structures andthe CMOS structures. These differences are on the order of the height ofthe gate electrodes of CMOS devices, typically in the range from about100 to about 250 nm. The height of at least one type of structure istypically adjusted with an accompanying compromise in the deviceperformance.

As disclosed in the U.S. Pat. No. 6,780,695, Chen et al. circumvents theproblem of height differences between the device types by depositing asacrificial polysilicon layer in a bipolar transistor area concurrentlywith a deposition of a gate polysilicon in the CMOS device area. Theoverall structure is planarized with a polysilicon placeholder material.A bipolar transistor is formed by removing the sacrificial polysiliconto expose an active silicon region, forming an intrinsic base and anemitter pedestal, and then forming an extrinsic base that is confinedwithin the opening of the sacrificial polysilicon layer. While Chen etal., enables an integration scheme for high performance BiCMOS circuit,the complexity of the process increases by the introduction ofadditional steps, notably, the deposition of polysilicon placeholdermaterial and planarization, deposition of a polysilicon layer over abase oxide and subsequent planarization utilizing an additionallithographic patterning.

Therefore, there exists a need to enable a high performance bipolartransistor with the benefits of self-aligned raised extrinsic base in aBiCMOS structure that contains at least one CMOS device.

There also exists a need to provide a high performance BiCMOS structurewith minimum process complexity without compromising the performance ofeither the bipolar transistor or CMOS devices.

SUMMARY OF THE INVENTION

The present invention addresses the needs described above by providingstructures and methods for a high performance BiCMOS circuit in whichbipolar transistors with self-aligned raised extrinsic base are formedwith CMOS devices.

The present invention also provides structures and methods in which aBiCMOS circuit is formed with less process complexity compared to theprior art, especially without utilizing chemical mechanicalplanarization (CMP) during the manufacture of the bipolar transistors.

According to the present invention, a semiconductor structure isdisclosed which comprise:

a semiconductor substrate;

a collector located in the semiconductor substrate;

shallow trench isolation (STI) adjoining and surrounding the collector;

at least one pad layer located directly on the STI;

an intrinsic base layer located directly on the collector and directlyon the at least one pad layer;

an emitter located directly on the intrinsic base layer;

an extrinsic base layer self-aligned to the emitter and directlycontacting the intrinsic base layer; and

a mesa structure with substantially planar and vertical sidewallsurfaces, wherein each of the sidewall surfaces contains the at leastone pad layer, the intrinsic base layer and the extrinsic base layer.

The above semiconductor structure can be integrated into a BiCMOSstructure that contains at least one CMOS device located on the samesemiconductor substrate. The at least one pad layer may comprise a stackof a pad oxide layer and a pad nitride layer. The extrinsic base maycomprise a semiconductor material selected from the group that consistsof doped silicon germanium alloy, doped silicon, doped silicon carbonalloy, and doped silicon germanium carbon alloy.

The above semiconductor structure may further comprise:

a first spacer located outside the emitter and contacting the emitterand the intrinsic base layer;

a second spacer contacting the first spacer, the emitter, and theextrinsic base layer; and

a third spacer containing the second spacer, the extrinsic base layer,and the second spacer layer.

In this case, preferably, the first spacer comprises a silicon oxide,the second spacer comprises a silicon nitride, and the third spacercomprises a silicon oxide.

In one aspect of the present invention, at least one pad layer isincluded in the mesa structure. The height of the intrinsic base layerand the height of the extrinsic base layer outside the activesemiconductor area are raised due to the at least one pad layer. Thiscompensates for the differential between the high growth rate of anepitaxial intrinsic base layer on the active semiconductor area and thelow growth rate of a polycrystalline intrinsic base layer outside theactive semiconductor area. Preferably, the thickness of the at least onepad layer is adjusted to match the height of the top surface of thepolycrystalline intrinsic base layer within the bipolar transistor area.

Two embodiments for fabricating the semiconductor structure above aredisclosed herein to demonstrate practicability. However, the presentinvention is not necessarily limited by the two embodiments. Shallowtrench isolation and a subcollector layer are formed first. Typically, acollector and a subcollector contact for each bipolar transistor to bebuilt are also formed. At least one active semiconductor area, which isan area of exposed epitaxial (single-crystalline) semiconductor surfacesurrounded by STI, is prepared in a bipolar device area. According toboth embodiments, an intrinsic base layer is formed using the followingcommon steps:

forming at lease one pad layer over an active semiconductor area;

forming an opening in the at least one pad layer to expose the activesemiconductor area; and

depositing an intrinsic base layer directly on the active semiconductorarea.

According to the first embodiment of the present invention, the abovesteps are followed by the following steps:

forming at least one emitter pedestal layer on the intrinsic base layer;

forming an emitter pedestal directly on the intrinsic base layer;

selectively depositing an extrinsic base layer on exposed portions ofthe intrinsic base layer;

forming a base cap dielectric layer on the extrinsic base layer; and

forming a mesa structure with substantially planar and vertical sidewallsurfaces, wherein each of the sidewall surfaces contains the at leastone pad layer, the intrinsic base layer and the extrinsic base layer.

According to the first embodiment, a second spacer is formed after theselective deposition of the extrinsic base layer.

According to the second embodiment of the present invention, the commonsteps above are followed by the following steps:

forming a pedestal etch stop layer on the intrinsic base layer;

forming at least one emitter pedestal layer on the pedestal etch stoplayer;

forming an emitter pedestal directly on the intrinsic base layer;

removing portions of the pedestal etch stop layer that are not coveredby the emitter pedestal;

selectively depositing an extrinsic base layer on exposed portions ofthe intrinsic base layer; and

forming a mesa structure with substantially planar and vertical sidewallsurfaces, wherein each of the sidewall surfaces contains the at leastone pad layer, the intrinsic base layer and the extrinsic base layer.

According to the second embodiment of the present invention, a secondspacer is formed prior to the selective deposition of the extrinsic baselayer.

In both embodiments, the extrinsic base layer is selectively depositedafter the formation of an emitter pedestal. Therefore, the extrinsicbase is self-aligned to the emitter. The process steps employed ineither embodiments of the present invention are less both in number andin complexity compared to methods known in the prior art for highperformance BiCMOS structures. Especially, no chemical mechanicalpolishing is required between the formation of the intrinsic base layerand the formation of the mesa structure, which is the last step uniqueto the fabrication of bipolar devices.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-4 are sequential cross-sectional views of an exemplary BiCMOSstructure during common process steps according to the first and thesecond embodiments of the present invention.

FIGS. 5-15 are sequential cross-sectional views of an exemplary BiCMOSstructure according to the first embodiment of the present invention.

FIG. 16 is a top down view of the exemplary BiCMOS structure in FIG. 15according to the first embodiment of the present invention. The dottedline X-X′ represents the plane of the cross-sectional view in FIG. 15.

FIGS. 17-29 are sequential cross-sectional views of an exemplary BiCMOSstructure according to the second embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention, which provides a bipolar transistor having araised extrinsic base formed by selective epitaxy and a mesa structureincluding the base and methods of fabricating the same, will now bedescribed in more detail by referring to the accompanying drawings. Itis noted that like and corresponding elements are referred to by likereference numerals.

While the present invention may be practiced to fabricate a bipolartransistor structure without any CMOS devices on the same semiconductorsubstrate, the benefits of the present invention are maximized whenpracticed for a BiCMOS structure that contains CMOS devices. For thepurposes of describing the present invention, an exemplary BiCMOSstructure comprising a CMOS transistor and a bipolar transistor isemployed. Applying the present invention to semiconductor structures tointegrated circuits with multiple CMOS devices and bipolar devices isstraightforward.

Referring to the vertical cross-sectional view of FIG. 1, a BiCMOSstructure with a bipolar device area B, and a CMOS device area C, isprovided. Preferably, shallow trench isolation 20 is formed in asemiconductor substrate 10. The semiconductor substrate may comprise asemiconductor material such as silicon, silicon germanium alloy, siliconcarbon alloy, or silicon germanium carbon alloy. The doping of thesemiconductor substrate may vary for optimal device performance.

A collector 30, a subcollector contact 31, and a CMOS device, which inthis example is a MOSFET, are thereafter formed. The doping type of thecollector is determined by the type of the bipolar transistor, i.e.,n-type in an NPN transistor or p-type in a PNP transistor. The dopingconcentration of the collector 30 is in the range from about1.0×10¹⁸/cm³ to about 1.0×10²¹/cm³, and preferably from about1.0×10¹⁹/cm³ to about 1.0×10²⁰/cm³. The thickness of the collector 30 isin the range from about 0.2 micron to about 1.5 micron, and preferablyin the range from about 0.3 micron to about 0.5 micron. The dopingprofile and the thickness of the collector 30 are optimized fortransistor performance. The subcollector 31 and the subcollector contact31 are heavily doped with the same type dopants as the collector 30,typically at a concentration on the order of 1.0×10²¹/cm³. The MOSFETcomprises a gate dielectric 41, a gate conductor 42, gate spacers 43,and source and drain regions 44. This structure has an activesemiconductor area within the bipolar device area B. The activesemiconductor area A is an area of exposed single-crystallinesemiconductor surface over the collector 30, and is surrounded by an STI20. The surface of the active area A preferably comprises the samematerial as the substrate 10.

As shown in FIG. 2, at least one pad layer (51, 52) is deposited overthe top surface of the semiconductor structure above. The at least onepad layer (51, 52) covers both the bipolar device area B and the CMOSdevice area C and directly contacts the active semiconductor area A, theSTI 20, the top of the subcollector contact 31, and the gate conductor42, the spacers 43, and the source and drain regions 44 of the MOSFET.The thickness of the at least one pad layer (51, 52) is preferablyoptimized to minimize topographical height variations of an intrinsicbase layer and an extrinsic base layer within the bipolar device area Bas they are formed subsequently. The thickness of the at least one padlayer (51, 52) is in the range from about 20 nm to about 200 nm, andpreferably in the range from about 50 nm to about 110 nm. Preferably,the at least one pad layer (51, 52) comprises a stack of a first padlayer 51 and a second pad layer 52. Preferably, the first pad layer 51is a silicon oxide layer and the second pad layer 52 is a siliconnitride layer. The thickness of the first pad layer is in the range fromabout 10 nm to about 100 nm, and preferably in the range from about 20nm to about 50 nm. The thickness of the second pad layer is in the rangefrom about 30 nm to about 60 nm.

As shown in FIG. 3, the at least one pad layer is thereafterlithographically patterned and removed from over the activesemiconductor area A such that the edge of the opening O is locatedwithin the STI 20 that surrounds the active semiconductor area A.

Referring to FIG. 4, after suitable surface cleaning to facilitateepitaxial growth of silicon or silicon alloy on the active semiconductorarea, an intrinsic base layer 60 is formed over the top surface ofpreexisting semiconductor structure. The intrinsic base layer 60comprises an epitaxial intrinsic base layer 60A and a polycrystallineintrinsic base layer 60B. The epitaxial intrinsic base layer 60A isformed with a high growth rate on and over the active semiconductor areaA with an epitaxial alignment to the underlying collector 30. Thepolycrystalline intrinsic base layer 60B is formed with a low growthrate outside the active semiconductor area A wherein no epitaxialseeding surface is available, that is, over the STI 20 and over the atleast one pad layer (51, 52). The boundaries between the epitaxialintrinsic base layer 60A and the polycrystalline intrinsic base layer Bare marked with dotted lines in FIG. 4. Also, the edges of the epitaxialintrinsic base layer are typically form facets as shown in FIG. 4.

The intrinsic base layer 60 comprises a silicon-containing semiconductormaterial. Preferably the intrinsic base layer 60 comprises p-dopedsilicon, n-doped silicon, p-doped silicon germanium alloy, n-dopedsilicon germanium alloy, p-doped silicon carbon alloy, n-doped siliconcarbon alloy, p-doped silicon germanium carbon alloy, n-doped silicongermanium carbon alloy, or any other semiconducting doped silicon alloy.The ratio of the thickness of the epitaxial intrinsic layer 60A to thethickness of the polycrystalline intrinsic base layer 60B depends on thecomposition of these layers, the reactant flow, and the depositiontemperature and is typically in the range from about 1.1 to about 3.0,and more typically in the range from 1.3 to about 1.8. The thickness ofthe epitaxial intrinsic base layer 60B is in the range from about 40 nmto about 400 nm, and preferably in the range from about 80 nm to about200 nm. The doping type of the intrinsic base layer 60 is determined bythe type of the bipolar transistor, i.e., p-type in an NPN transistor orn-type in a PNP transistor. The peak doping concentration of theepitaxial intrinsic base layer 60A is in the range from about1.0×10¹⁷/cm³ to about 1.0×10²⁰/cm³, and preferably from about1.0×10¹⁸/cm³ to about 1.0×10¹⁹/cm³. The peak of the dopant profile islocated at a depth, measured from the top surface of the epitaxialintrinsic base layer 60A, from 0 nm to about 50 nm, and preferably fromabout 10 nm to about 30 nm. The doping profile and the thickness of theintrinsic base layer are optimized for performance. The dopingconcentration of the polycrystalline intrinsic base layer 60B is of thesame order of magnitude, and typically varies by less than a factor of 3compared to that of the epitaxial intrinsic base layer 60A.

Preferably, the thickness of the at least one pad layer (51, 52) areadjusted so that the height of the top surface of the epitaxialintrinsic base layer 60A and the height of the top surface of thepolycrystalline intrinsic base layer 60B are not substantiallydifferent, for example, different by less than 50 nm, and mostpreferably, substantially the same within the bipolar device area B.

According to the first embodiment of the present invention, at least oneemitter pedestal layer (71, 72) is formed directly on the intrinsic baselayer 60 thereafter as shown in FIG. 5. The at least one emitterpedestal layer (71, 72) is formed over the bipolar device area B andover the CMOS device area C. Preferably, the at least one emitterpedestal layer (71, 72) comprises a first emitter pedestal layer 71 anda second emitter pedestal layer 72. The thickness of the first emitterpedestal layer is in the range from 5 nm to about 20 nm, and morepreferably from about 10 nm to about 15 nm. The thickness of the secondemitter pedestal layer is in the range from 20 nm to about 100 nm, andmore preferably from about 30 nm to about 50 nm. Preferably, the firstemitter pedestal layer is a silicon oxide layer and the second emitterpedestal layer is a silicon nitride layer.

An emitter pedestal is formed by applying a first photoresist 75 andlithographically patterning and etching the at least one emitterpedestal layer (71, 72) with a reactive ion etch (RIE) as shown in FIG.6. The RIE may be used alone or in combination with a wet etch to exposethe underlying intrinsic base layer 60. If two emitter pedestal layersare used, RIE may remove the exposed portion of the second emitterpedestal layer 72 and a wet etch may be employed to remove the exposedportion of the first emitter pedestal layer 71 to produce a structure inFIG. 6.

Thereafter, the first photoresist 75 is removed. Appropriate surfaceclean, for example, a wet etch in hydrofluoric acid (HF), may beperformed at this point.

According to the first embodiment of the present invention, an extrinsicbase layer 62 is formed by a selective deposition of silicon-containingmaterial as shown in FIG. 7. The silicon-containing material may besilicon, silicon germanium alloy, silicon carbon alloy, or silicongermanium carbon alloy. Preferably, the silicon-containing material isheavily doped with the same type of dopants as the intrinsic base layer60. The doping concentration of the extrinsic base layer 62 is in therange from about 1.0×10¹⁹/cm³ to about 1.0×10²²/cm³, and preferably fromabout 1.0×10²⁰/cm³ to about 1.0×10²¹/cm³. The thickness of the extrinsicbase layer 62 is in the range from about 10 nm to about 150 nm, andpreferably in the range from about 20 nm to about 50 nm. The ratio ofthe doping concentration of extrinsic base layer 62 to the peak dopingconcentration of epitaxial intrinsic base layer 60A is in the range fromabout 10 to about 10,000, and preferably from about 300 to about 3000.

The selective deposition of the extrinsic base layer 62 is “selective”as implied in the name. In the selective deposition process “of theextrinsic base layer 62”, a semiconductor material nucleates anddeposits on a semiconductor surface , that is, on the surface of theintrinsic base layer 60, while not depositing on insulator surfaces suchas the surface of the emitter pedestal that comprises the at least oneemitter pedestal layer (71, 72). Therefore, the extrinsic base layer 62grows only from the exposed intrinsic base layer 60 and does not growfrom the surfaces of the emitter pedestal. The extrinsic base layer 62is not formed over the top surface of the emitter pedestal.

Selectivity of the deposition process may be provided by an etchant suchas hydrogen chloride (HCl) in the reactant stream or by germanium sourcesuch as germane (GeH₄) or digermane (Ge₂H₆). If the extrinsic base layer62 does not contain germanium, a separate etchant needs to be suppliedinto a reaction chamber. If the extrinsic base layer 62 containsgermanium and therefore, a germanium source gas is supplied into thereaction chamber, supplying an additional etchant is optional.

The portion of the extrinsic base layer 62 that is formed over thepolycrystalline intrinsic base layer 60B is always polycrystalline,i.e., not epitaxial, since a lattice structure for an epitaxialalignment is not provided by a surface below. The portion of theextrinsic base layer 62 that is formed over the epitaxial intrinsic baselayer 60 may be epitaxial or polycrystalline depending on depositionconditions and surface preparation. The selective deposition may beperformed in a single wafer processing chamber or in a batch furnace.The deposition temperature is in the range from about 450° C. to about1,000° C., and preferably from about 600° C. to about 900° C. Theprocess pressure may vary between single wafer processing chambers andbatch furnaces. Typical process pressure is in the range from about 1Torr to 200 Torr, more preferably from about 40 Torr to about 80 Torr ina singe wafer processing chamber and is in the range from about 1 mTorrto 5 Torr, more preferably from about 5 mTorr to about 200 mTorr in abatch furnace.

Referring to FIG. 8, a base cap dielectric layer 64 is formed directlyon the underlying extrinsic base layer 62. The base cap layer 64 isselectively formed on the extrinsic base layer but is not formed on theemitter pedestal. The thickness of the base cap dielectric layer 64 isin the range from about 10 nm to about 100 nm, and preferably from about30 nm to about 80 nm. The base cap dielectric layer 64 may be formed bythermal oxidation or by selective deposition of dielectric, such asselective deposition of silicon oxide. If the base cap dielectric layer64 is formed by thermal oxidation, a portion of the underlying extrinsicbase layer 62 is consumed and converted to silicon oxide. Thermaloxidation may be performed either at an atmospheric pressure or at ahigher pressure. If an atmospheric oxidation process is used, theoxidation process temperature is in the range from about 700° C. toabout 800° C. Preferably, a high pressure oxidation (HiPOx) process isemployed to reduce the temperature, in which case the oxidation processtemperature is in the range from about 575° C. to about 675° C. Typicalprocess pressure for a HiPOx process is in the range from about 10 atmto about 20 atm.

Referring to FIG. 9, a portion of the at least one emitter pedestallayer (71, 72) is removed afterwards. Preferably, the at least oneemitter pedestal layer (71, 72) comprises a stack of a first emitterpedestal layer 71 and the second emitter pedestal layer 72. Preferably,the first emitter pedestal layer is a silicon oxide and the secondemitter pedestal layer is a silicon nitride as noted above. In thiscase, the second emitter pedestal layer is removed to expose an innerextrinsic base sidewall 66 as shown in FIG. 9.

By a conformal deposition of a dielectric layer and a RIE, a spacer, tobe identified as a “second spacer” 81 herebelow, is formed along theinner wall of the extrinsic base layer 62 and also along the inner wallof the base cap dielectric layer 64 as shown in FIG. 10. The secondspacer 81 is a contiguous structure that is topologically homomorphic toa torus. The two inner walls are coincident as seen from above.Preferably, the second spacer 81 comprises a silicon nitride.

As shown in FIG. 11, another portion of the at least one emitterpedestal layer (71, 72) is removed to expose the top surface of aportion of the intrinsic base layer 60, specifically, the top surface ofa portion of the epitaxial intrinsic base layer 60A. In the preferredversion described above wherein the first emitter pedestal layer 71 isan oxide and the second emitter pedestal layer 72 is a nitride, thefirst emitter pedestal layer 71 is removed by a wet etch, for example ina hydrofluoric acid (HF) solution. The preferred thickness ranges forthe first emitter pedestal layer 71 and the base oxide dielectric caplayer 64 are such that a substantial portion of the base oxidedielectric cap layer 64 still remains after such a wet etch. The wetetch creates an undercut on the remainder of the first emitter pedestallayer 71 such that the remnant of the first emitter pedestal layer 71forms another spacer, to be identified as a “first spacer” 71′herebelow, around the opening within the second spacer 81. The firstspacer 71′ is a contiguous structure that is topologically homomorphicto a torus. The first spacer 71′ contacts the intrinsic base 60 and theextrinsic base 62. The second spacer 81 contacts the first spacer 71′,the extrinsic base 62, and the base cap dielectric layer 64.

Referring to FIG. 12, a doped emitter layer 90 is deposited. The dopedemitter layer 90 comprises a doped silicon-containing material and isdoped with dopants of the same type as the dopants in the collector,i.e., n-type in an NPN transistor or p-type in a PNP transistor. Thedoping concentration of the doped emitter layer 90 is in the range fromabout 1.0×10²⁰/cm³ to about 1.0×10²²/cm³, and preferably from about3.0×10²⁰/cm³ to about 1.0×10²¹/cm³. The thickness of the doped emitterlayer 90 is in the range from about 80 nm to about 300 nm, andpreferably from about 100 nm to about 200 nm. Preferably, epitaxialalignment with the underlying intrinsic base layer 60 is avoided byforming a thin thermal oxide at the interface between the intrinsic baselayer 60 and the doped emitter layer 90.

Referring to FIG. 13, a second photoresist 95 is thereafter applied overthe top surface of the semiconductor structure above andlithographically patterned. This is followed by a pattern transfer intothe doped emitter layer 90 to form an emitter 91. The emitter 91completely covers the underlying second spacer 81 as seen from above.The sidewall of the emitter 91 overlies the base cap dielectric layer64. The doped emitter layer 90 is removed from all other area thatexcludes the emitter 91.

Referring to FIG. 14, a third photoresist 96 is applied over the topsurface of the semiconductor structure above and lithographicallypatterned to define a mesa area that includes at least the area of theemitter 91 and the active semiconductor area A. This is followed by apattern transfer into the base cap dielectric layer 64, the extrinsicbase layer 62, the intrinsic base layer 60, and optionally, a portion ofthe at least one pad layer (51, 52) through a RIE. In a preferredversion of the first embodiment, the first pad layer 51 is a siliconoxide and the second pad layer 52 is a silicon nitride. In this case,the RIE may remove exposed portions of the second pad layer 52 but doesnot remove the first pad layer 51. Alternatively, the RIE may remove allof the exposed pad layers (51, 52).

The photoresist 96 is thereafter removed. Either a wet etch or a RIE isperformed to remove the remaining portion of the at least one pad layer(51, 52). The resultant structure is shown in FIG. 15. In the preferredversion of the first embodiment above, the remaining portion of thefirst pad layer 51 is removed from all exposed area. This leaves anotherspacer, to be designated a “third spacer” 64′ herebelow, beneath theemitter 91. The third spacer 64′ is a contiguous structure that istopologically homomorphic to a torus. The third spacer 64′ comprises thesame material as the base cap dielectric layer 64, and is preferably asilicon oxide.

The structure in FIG. 15 has a mesa structure that comprises anextrinsic base layer 62, an intrinsic base layer 60, and at least onepad layer (51, 52). The sidewalls 99 of the mesa structure aresubstantially planar and vertical. FIG. 16 shows a top-down view of thestructure in FIG. 15. The dotted line X-X′ represents the plane of thecross-sectional view in FIG. 15. As seen from above, the sidewalls ofindividual layers, that is, the sidewalls of the extrinsic base layer62, the sidewalls of the intrinsic base layer 60, and the sidewalls ofthe at least one pad layer (51, 52) are coincident. Seen from anarbitrary angle, the sidewalls of the individual layers thus formsubstantially planar and vertical sidewalls of the mesa structure.

According to the second embodiment of the present invention, alternatemethods are used to form a semiconductor structure similar to those thatare shown in FIGS. 15-16. The fabrication process is the same up to thestructure corresponding to FIG. 4. Instead of forming at least oneemitter pedestal layer (71, 72) as shown in FIG. 5, a pedestal etch stoplayer 171 is formed on the intrinsic base layer 60 as shown in FIG. 17according to the second embodiment. The pedestal etch stop layer 171 isa dielectric layer. Preferably, the pedestal etch stop layer 171 is asilicon oxide layer. The thickness of the pedestal etch stop layer is inthe range from about 5 nm to about 50 nm, and preferably in the rangefrom 10 nm to about 30 nm.

Thereafter, at least one emitter pedestal layer (172, 173, 174) isformed on the pedestal etch stop layer. In a preferred version of thesecond embodiment, the at least one emitter pedestal layer (172, 173,174) comprise a first emitter pedestal layer 172, a second emitterpedestal layer 173, and a third emitter pedestal layer 174. In a mostpreferred version of the second embodiment, the first emitter pedestallayer 172 is a polysilicon layer, the second emitter pedestal layer 173is a silicon nitride layer, and the third emitter pedestal layer 174 isa silicon oxide layer. In this case, the first emitter pedestal layer172 has a thickness in the range from about 30 nm to about 150 nm, andpreferably from about 50 nm to about 100 nm; the second emitter pedestallayer 173 has a thickness in the range from about 10 nm to about 80 nm,and preferably from about 20 nm to about 50 nm; and the third emitterpedestal layer 174 has a thickness in the range from about 5 nm to about50 nm, and preferably from about 10 nm to about 30 nm.

An emitter pedestal is formed by applying a first photoresist 75 andlithographically patterning and etching the at least one emitterpedestal layer (172, 173, 174) with a reactive ion etch (RIE) as shownin FIG. 18. The RIE stops on the pedestal etch stop layer 171. Theemitter pedestal is defined by the remnant of the at least one emitterpedestal layer (172, 173, 174) underneath the patterned firstphotoresist 75 after removing the exposed portions of the at least oneemitter pedestal layer (172, 173, 174). Thereafter, the firstphotoresist 75 is removed.

An outer pedestal spacer 181 is formed by a conformal deposition of adielectric layer and a RIE as shown in FIG. 19. The outer pedestalspacer 181 contacts the emitter pedestal and a portion of the topsurface of the pedestal etch stop layer 171. In the most preferredversion of the second embodiment, the outer pedestal spacer 181 layer isa silicon nitride.

Thereafter, the exposed portions of the pedestal etch stop layer 171 andoptionally a portion of the at least one emitter pedestal layer (172,173, 174) are removed as shown in FIG. 20. In the most preferred versionof the second embodiment, both the pedestal etch stop layer 171 and thethird emitter pedestal layer 174 are silicon oxides and are preferablyremoved by a wet etch, for example in a hydrofluoric acid (HF) solution.In this version, an undercut is formed in the remaining pedestal etchstop layer 171 underneath the outer pedestal spacer 181. Also, such anwet etch may be utilized to clean the surface of the exposed top surfaceof the intrinsic base layer 60.

According to the second embodiment of the present invention, anextrinsic base layer 62 is formed by selective deposition ofsilicon-containing material as shown in FIG. 21. According to the secondembodiment, the specifications for the extrinsic base layer 62 and thesilicon-containing material therein, including composition, doping,thickness, and crystalline structure, are identical to that according tothe first embodiment, as described in passages accompanying FIG. 7. Thespecifications for the selective deposition process for the extrinsicbase layer 62 are identical to that according to the first embodiment,as described in passages accompanying FIG. 7 as well. Since thedeposition process for the extrinsic base layer 62 is selective, theextrinsic layer 62 is not formed over the top surface of the emitterpedestal.

Referring to FIG. 22, a base cap dielectric layer 64 is formed directlyon the underlying extrinsic base layer 62. The specifications for thestructure and formation process for the base cap dielectric layer 64according to the second embodiment is identical to those according tothe first embodiment as described in passages accompanying FIG. 8.

Referring to FIG. 23, a portion of the outer pedestal spacer 181 andoptionally another portion of the at least one emitter pedestal layer(172, 173, 174) within the remaining emitter pedestal structure arethereafter removed preferably by wet etch. The remaining portion of theouter spacer layer 181, to be identified as a “second spacer” 181′herebelow, forms a shorter spacer contacting the extrinsic base layer62. The second spacer 181′ is a contiguous structure that istopologically homomorphic to a torus.

In the most preferred version of the second embodiment, both the outerpedestal spacer 181′ and the second emitter pedestal layer 172 aresilicon nitrides and are removed by a wet etch. The first emitterpedestal layer 172, which is a polysilicon layer, is exposed after thewet etch and the second spacer 181′ comprises silicon nitride.

Referring to FIG. 24, another portion of the at least one emitterpedestal layer (172, 173, 174) is removed as needed to expose thepedestal etch stop layer 171. In the most preferred version of thesecond embodiment, the first emitter pedestal layer 172, which is apolysilicon layer, is removed by a RIE or by a wet etch and exposes theunderlying pedestal etch stop layer, which is a silicon oxide in thisversion.

Referring to FIG. 25, a portion of the pedestal etch stop layer 171 isremoved to expose a top surface of the intrinsic base layer 60 withinthe active semiconductor area A. The removed portion include a portionof the pedestal etch layer 171 within the area surrounded by the innerwall of the second spacer 181′ as seen from above. The remnant of theetch stop layer 171, to be identified as a “first spacer” herebelow, isformed and contacts the intrinsic base layer 60 and the extrinsic baselayer 62. The first spacer 171′ has a continuous structure that istopologically homomorphic to a torus. In the most preferred version ofthe second embodiment, the pedestal etch stop layer 171′ is an oxide anda wet etch in a hydrofluoric acid (HF) solution or a RIE is employed toexpose the top surface of the intrinsic base layer 60. An undercut maybe formed below the second spacer 181′ in this version.

Referring to FIG. 26, a doped emitter layer 90 is deposited. Thespecifications for the structural and compositional aspects of the dopedemitter layer 90 according to the second embodiment is identical tothose according to the first embodiment as described in passagesaccompanying FIG. 12.

Referring to FIG. 27, a second photoresist 95 is thereafter applied overthe top surface of the semiconductor structure above andlithographically patterned. This is followed by a pattern transfer intothe doped emitter layer 90 to form an emitter 91. The emitter 91 coversthe underlying second spacer 181′ completely as seen from above. Thesidewall of the emitter 91 overlies the base cap dielectric layer 64.The doped emitter layer 90 is removed from all other area that excludesthe emitter 91.

Referring to FIG. 28, a third photoresist 96 is applied over the topsurface of the semiconductor structure above and lithographicallypatterned to define a mesa area that includes at least the area of theemitter 91 and the active semiconductor area A. This is followed by apattern transfer into the base cap dielectric layer 64, the extrinsicbase layer 62, the intrinsic base layer 60, and optionally, a portion ofthe at least one pad layer (51, 52) through a RIE. In a preferredversion of the first embodiment, the first pad layer 51 is a siliconoxide and the second pad layer 52 is a silicon nitride. In this case,the RIE may remove exposed portions of the second pad layer 52 but doesnot remove the first pad layer 51. Alternatively, the RIE may remove allof the exposed pad layers (51, 52). Except for minor differences inidentification numerals for the various spacers, i.e., the first spacer,the second spacer, and the third spacer, in their dimensions and in thedimensions of the emitter 91, the structure of FIG. 28 is identical tothat in FIG. 14.

The photoresist 96 is thereafter removed. Either a wet etch or a RIE isperformed to remove the remaining portion of the at least one pad layer(51, 52). The resultant structure is shown in FIG. 29. In the preferredversion of the first embodiment above, the remaining portion of thefirst pad layer 51 is removed from all exposed area. This leaves anotherspacer, to be designated a “third spacer” 64′ herebelow, beneath theemitter 91. The third spacer 64′ is a contiguous structure that istopologically homomorphic to a torus. The third spacer 64′ comprises thesame material as the base cap dielectric layer 64, and is preferably asilicon oxide.

The structure in FIG. 29 has a mesa structure that comprises anextrinsic base layer 62, an intrinsic base layer 60, and at least onepad layer (51, 52). The sidewalls 99 of the mesa structure aresubstantially planar and vertical. Except for minor differences inidentification numerals for the various spacers and in their dimensions,the structure of FIG. 29 is identical to that in FIG. 15. Therefore, thestructure of FIG. 29 has the same top down view as the structure in FIG.15, which is FIG. 16 except for minor differences in the dimensions ofthe emitter 91. As seen from above, the sidewalls of individual layers,that is, the sidewalls of the extrinsic base layer 62, the sidewalls ofthe intrinsic base layer 60, and the sidewalls of the at least one padlayer (51, 52) are coincident. Seen from an arbitrary angle, thesidewalls of the individual layers thus form substantially planar andvertical sidewalls of the mesa structure.

Throughout the accompanying figures, all the layers formed by depositionor oxidation in the bipolar device area B are also formed in the CMOSdevice area C. Specifically, the at least one pad layer, the intrinsicbase layer, at least one emitter pedestal layer, extrinsic base layer,and the base cap layer according to the first embodiment of the presentinvention are formed both in the bipolar device area B and in the CMOSdevice area C. Also, the at least one pad layer, the intrinsic baselayer, the pedestal etch stop layer, at least one emitter pedestallayer, extrinsic base layer, and the base cap layer according to thesecond embodiment of the present invention are formed both in thebipolar device area B and in the CMOS device area C. One aspect of thepresent invention is that these layers protect the CMOS devices in theCMOS device area C throughout the processes that form bipolartransistors.

While this invention has been particularly shown and described withrespect to preferred embodiments thereof, it will be understood by thoseskilled in the art that the foregoing and other changes in forms anddetails may be made without departing from the spirit and scope of thepresent invention. It is therefore intended that the present inventionnot be limited to the exact forms and details described and illustrated,but fall within the scope of the appended claims.

What is claimed is:
 1. A method of fabricating a semiconductorstructure, comprising: forming at lease one pad layer over an activesemiconductor area; forming an opening in said at least one pad layer toexpose said active semiconductor area; depositing an intrinsic baselayer directly on said active semiconductor area; forming a pedestaletch stop layer on said intrinsic base layer; forming at least oneemitter pedestal layer on said pedestal etch stop layer; forming anemitter pedestal directly on said intrinsic base layer; removingportions of said pedestal etch stop layer that are not covered by saidemitter pedestal; selectively depositing an extrinsic base layer onexposed portions of said intrinsic base layer; and forming a mesastructure with substantially planar and vertical sidewall surfaces,wherein each of said sidewall surfaces contains said at least one padlayer, said intrinsic base layer and said extrinsic base layer.
 2. Themethod of claim 1, further comprising: forming at least one spacer on asidewall of said emitter pedestal after said forming of said emitterpedestal; removing at least a portion of said emitter pedestal aftersaid selectively depositing of said extrinsic base layer; and forming anemitter directly on said intrinsic base layer and directly on said atleast one spacer prior to said forming of said mesa structure.
 3. Themethod of claim 2, further comprising: forming a base cap dielectriclayer on said extrinsic base layer prior to said forming of saidemitter; and patterning said base cap dielectric layer, said extrinsicbase layer, and said intrinsic base layer such that the patterned areainclude said active semiconductor area.
 4. The method of claim 3,further comprising forming at least one CMOS device on a semiconductorsubstrate prior to said forming of said at least one pad layer, whereinsaid at least one pad layer is formed over said at least one CMOSdevice; and removing said at least one pad layer from over said at leastone CMOS device.
 5. The method of claim 4, wherein said at least one padlayer, said intrinsic base layer, said pedestal etch stop layer, said atleast one emitter pedestal layer, said extrinsic base layer, and saidbase cap dielectric layer are formed over said at least one CMOS deviceand said extrinsic base layer is not formed over a top surface of saidemitter pedestal.
 6. The method of claim 5, wherein said at least onepad layer comprises a stack of a silicon oxide layer and a siliconnitride layer.
 7. The method of claim 5, wherein said intrinsic baselayer comprises a material selected from the group consisting of p-dopedsilicon, n-doped silicon, p-doped silicon germanium alloy, n-dopedsilicon germanium alloy, p-doped silicon carbon alloy, n-doped siliconcarbon alloy, p-doped silicon germanium carbon alloy, and n-dopedsilicon germanium carbon alloy.
 8. A semiconductor structure comprising:a semiconductor substrate; a collector located in said semiconductorsubstrate; shallow trench isolation (STI) adjoining and surrounding saidcollector; at least one pad layer located directly on said STI; anintrinsic base layer located directly on said collector and directly onsaid at least one pad layer; an emitter located directly on saidintrinsic base layer; an extrinsic base layer self-aligned to saidemitter and directly contacting said intrinsic base layer; and a mesastructure with substantially planar and vertical sidewall surfaces,wherein each of said sidewall surfaces contains said at least one padlayer, said intrinsic base layer and said extrinsic base layer.
 9. Thesemiconductor structure of claim 8, further comprising at least one CMOSdevice located on said semiconductor substrate.
 10. The semiconductorstructure of claim 9, wherein said at least one pad layer comprises astack of pad oxide layer and a pad nitride layer.
 11. The semiconductorstructure of claim 9, wherein said extrinsic base layer comprises asemiconductor material selected from the group that consists of a dopedsilicon germanium alloy, a doped silicon, a doped silicon carbon alloy,and a doped silicon germanium carbon alloy.
 12. The semiconductorstructure of claim 9, further comprising; a first spacer located outsidesaid emitter and contacting said emitter and said intrinsic base layer;a second spacer contacting said first spacer, said emitter, and saidextrinsic base layer. a third spacer containing said second spacer, saidextrinsic base layer, and said second spacer layer.
 13. Thesemiconductor structure of claim 12, wherein said first spacer comprisesa silicon oxide, said second spacer comprises a silicon nitride, andsaid third spacer comprises a silicon oxide.